The systems and techniques described herein were made in the performance of work under a NASA contract, and are subject to the provisions of Public Law 96-517 (35 USC 202) in which the Contractor has elected to retain title.
This application relates to opto-electronic oscillators and their applications.
An oscillating electrical signal may be used to carry information in either digital or analog form. The information can be imbedded in the electrical signal by a proper modulation, such as the amplitude modulation, the phase modulation, and other modulation techniques. The information in the electrical signal may be created in various ways, e.g., by artificially modulating the electrical carrier, or by exposing the electrical carrier to a medium which interacts with the carrier. Such signals may be transmitted via space or conductive cables or wires.
It is well known that an optical wave may also be used as a carrier to carry information in either digital or analog form by optical modulation. Such optical modulation may be achieved by, e.g., using a suitable optical modulator, to modulate either or both of the phase and amplitude of the optical carrier wave. Signal transmission and processing in optical domain may have advantages over the electrical counterpart in certain aspects such as immunity to electromagnetic interference, high signal bandwidth per carrier, and easy parallel transmission by optical wavelength-division multiplexing (WDM) techniques.
Certain devices and systems may be designed to have electrical-optical xe2x80x9chybridxe2x80x9d configurations where both optical and electrical signals are used to explore their respective performance advantages, conveniences, or practical features. Notably, opto-electronic oscillators (xe2x80x9cOEOsxe2x80x9d)are formed by using both electronic and optical components to generate oscillating signals in a range of frequencies, e.g., from the microwave spectral ranges to the radio-frequency (xe2x80x9cRFxe2x80x9d) spectral range. See, e.g., U.S. Pat. Nos. 5,723,856, 5,777,778, 5,929,430, and 5,917,179 for some examples of OEOs.
Such an OEO typically includes an electrically controllable optical modulator and at least one active opto-electronic feedback loop that comprises an optical part and an electrical part interconnected by an optical-to-electrical conversion element such as a photodetector. The opto-electronic feedback loop receives the modulated optical output from the modulator and converted it into an electrical signal to control the modulator. The loop produces a desired delay and feeds the electrical signal in phase to the modulator to generate and sustain both optical modulation and electrical oscillation when the total loop gain of the active opto-electronic loop and any other additional feedback loops exceeds the total loss. The generated oscillating signals can be tunable in frequency and have narrow spectral linewidths and low phase noise in comparison with the signals produced by other RF and microwaves oscillators. OEOs can be particularly advantageous over other oscillators in the high RF spectral ranges, e.g., frequency bands on the order of GHz and tens of GHz.
Techniques and devices of this application are in part based on the recognition that the long-term stability and accuracy of the oscillating frequency of an OEO may be desirable in various applications. Accordingly, this application discloses, among other features, mechanisms for stabilizing the oscillating frequency of an OEO with respect to or at a reliable frequency reference to provide a highly stable signal. In addition, the absolute value of the oscillating frequency of the OEO can be determined with high accuracy or precision. The reliable frequency reference may be, for example, a reference frequency defined by two energy levels in an atom. Thus, such an OEO can be coupled to and stabilized to the atomic reference frequency to operate as an atomic clock.
In one exemplary implementation, a device according to this application may include an opto-electronic oscillator and an atomic reference module that are coupled to each other. The opto-electronic oscillator may include an opto-electronic loop with an optical section and an electrical section and operable to generate an oscillation at an oscillation frequency. The atomic reference module may be coupled to receive and interact with at least a portion of an optical signal in the optical section to produce a feedback signal. The opto-electronic oscillator is operable to respond to this feedback signal to stabilize the oscillation frequency with respect to an atomic frequency reference in the atomic reference module.
In another exemplary implementation, a device according to this application may include an optical modulator, an opto-electronic loop, a frequency reference module, and a feedback module. The optical modulator is operable to modulate an optical carrier signal at a modulation frequency in response to an electrical modulation signal to produce modulation bands in the optical carrier signal. The opto-electronic loop has an optical section coupled to receive a first portion of the optical carrier signal, and an electrical section to produce the electrical modulation signal according to the first portion of the optical carrier signal. The opto-electronic loop causes a delay in the electrical modulation signal to provide a positive feedback to the optical modulator. The frequency reference module has an atomic transition in resonance with a selected modulation band among the modulation bands and is coupled to receive a second portion of the optical carrier signal. The second portion interacts with the atomic transition to produce an optical monitor signal. The feedback module is operable to receive the optical monitor signal and to control the optical modulator in response to information in the optical monitor signal to lock the modulation frequency relative to the atomic transition.
This application also discloses various methods for operating or controlling opto-electronic oscillators. In one method, for example, a coherent laser beam is modulated at a modulation frequency to produce a modulated optical beam. Next, a portion of the modulated optical beam is transmitted through an optical delay element to cause a delay. The portion of the optical signal from the optical delay element is converted into an electrical signal. This electrical signal is then used to control the modulation of the coherent laser beam to cause an oscillation at the modulation frequency. A deviation of the modulation frequency from an atomic frequency reference is then obtained. The modulation of the coherent laser beam is then adjusted to reduce the deviation.
These and other implementations of the devices and techniques of this application are now described in greater details as follows.